Clutches are well known devices that are frequently employed in machinery to connect rotatively driven mechanisms to a source of rotational power. Typically, a clutch includes an input shaft, an output shaft and some mechanism to selectively couple the input shaft to the output shaft. When the clutch is engaged, the input shaft is connected to the output shaft to rotatably drive the driven mechanism. When the clutch is disengaged the input shaft is disconnected from the output shaft. There are other different types of clutches, including friction clutches.
One well known type of clutch is a wrap spring clutch. The primary parts of a wrap spring clutch include an input hub or shaft, an output hub or shaft, and a coiled drive spring for selectively causing the input hub to rotatably drive the output hub. Wrap spring clutches are used for a variety of applications, including riveting machines, copiers, stitching machines and conveyors. The purpose of selectively driving the output hub is accomplished by providing the input hub and the output hub with adjacent, axially aligned cylindrical surfaces. Portions of the drive spring are disposed about each of these cylindrical surfaces. The drive spring has a relaxed inner diameter which is slightly smaller than the outer diameter of the cylindrical surfaces of the input and output hubs. Thus, as is well known in the art, when the input hub is rotated in a first direction, the drive spring is wrapped tightly about the co-axially oriented cylindrical surfaces. As a result, the output hub is driven to rotate in the first direction with the input hub. When the input hub is rotated in a second direction, however, the drive spring is expanded about the co-axially oriented cylindrical surfaces. As a result, the output hub is not driven to rotate in the second direction with the input hub.
A frequent arrangement involving clutches, particularly wrap spring clutches, includes a solenoid arm to control the engagement and disengagement of the clutch. Control tangs at the end of the drive spring facilitate the expansion and contraction of the drive spring. The control tangs are fixed within a hollow cylindrical control collar disposed about the drive spring. The external surface of the control collar is fitted with several stops that are selectively engaged by the solenoid arm. When the control collar is engaged by the solenoid arm, the control tang is moved such that the drive spring expands about the input and output hubs, resulting in disengagement of the wrap spring clutch. When the collar is not engaged by the solenoid arm, the drive spring contracts about the input and output hubs, resulting in engagement of the clutch, and co-rotation of the input and output hubs.
A solenoid coil is typically used to operate the solenoid arm. As is well known, a solenoid coil includes an armature that is axially movable in response to the passing of an electric current passing through an electromagnetic coil. As a result, the solenoid arm can be selectively moved into and out of engagement with the control collar to control the operation of the clutch. The clutch, the solenoid arm and the solenoid coil are referred to collectively as a clutch assembly.
In a common mounting design for a clutch assembly, the clutch, the solenoid coil and the solenoid arm are all mounted on a clutch plate or other support member so that these components can be fixed relative to each other. The clutch plate can then be mounted on any desired base structure or support surface, such as a structural support or structural frame member associated with the machine in which the clutch is housed. A wrap spring clutch can include a brake hub extending along the axial centerline of the clutch. Mounted for rotation on the brake hub are the input hub and the output hub. The brake hub is rigidly attached to the clutch plate by any means, such as by bolting.
Operation of the clutch results in torque forces between the clutch and the clutch plate. The clutch plate must therefore be locked or tied to a nonrotatable element so that it doesn't rotate along with the clutch. It would seem that a simple solution to prevent unwanted rotation of the clutch plate would be to bolt or otherwise rigidly mount the clutch plate to a support surface, such as a structural support or structural frame member associated with the machine in which the clutch is housed. However, rigidly attaching the clutch plate to a relatively immovable surface leads to other problems. During operation of the clutch, the torque forces between the clutch and the clutch plate cause the clutch plate to be twisted or flexed relative to the clutch plate. If the clutch plate is rigidly tied or fixed to an immovable surface, the plate hub and the output hub faces will bind, causing premature wear and excessive heat generation.
This problem of binding has been solved in the past by mounting the clutch plate in an arrangement that allows the clutch plate to move relatively freely in different planar angles with respect to the support surface as needed to accommodate the torque forces of the clutch without binding. The angular movement is through a relatively small angular range, usually no greater than about 10 degrees. Even though angular movement of the clutch plate is permitted, the clutch plate must still be prevented from rotating in a direction around the longitudinal axis of the clutch.
To prevent rotation of the clutch plate vis-a-vis an immovable support surface, an antirotation hole or slot is incorporated in the clutch plate to allow a connection with a relatively fixed surface. Shoulder bolts, dowel pins or other types of bolts are passed through the antirotation slot and are rigidly connected to an immovable support surface to prevent rotation of the clutch plate. The antirotation slot is an orifice having greater dimensions than the diameter of the bolt rigidly connected to the support surface. The clutch plate is prevented from rotating by the contact between the bolt and the slot, but clutch plate can still move angularly to accommodate the torque forces between the clutch and clutch plate. As an alternative to the antirotation slot, antirotation connections to the edge of the clutch plate can also be used.
The use of an antirotation slot in the clutch plate is not without its problems, however. The repeated action of the loosely fitting bolt eventually wears or elongates the slot. This elongation affects the output stop position and causes the plate to bounce. This bouncing can result in "double cycling" of the clutch, where the stop cam bounces over the actuator causing the clutch to continue driving through to the next stop. If this condition becomes excessive, the stop cam will continue to ratchet over the actuator and the clutch will drive continuously. The increased play generated at the antirotation position on the plate can also cause the solenoid plunger to bounce, thereby causing the unit to prematurely fail. Further, as the play at the antirotation slot increases, the metal-to-metal noise grows to an undesirable level.
In the past, designs employing resilient materials around the pins or bolts have been used in an attempt to reduce the wear and noise associated with these clutches. While using resilient materials provides temporary benefits in reduced wear and diminished noise, the resilient material soon wears out and requires replacement, and therefore this is not a satisfactory solution.
It would be advantageous if a new mechanism could be developed to prevent the clutch plate from rotating, particularly where the mechanism allowed the clutch plate to undergo a minimal amount of angular play vis-a-vis the support surface to accommodate the forces between the clutch and the clutch plate. Such an improved mechanism would eliminate the double clutch cycling condition and the excessive noise associated with currently used antirotation slots having too much play.